Certain semiconductor devices, e.g., magnetic random access memory (MRAM) devices, use magnetic memory cells to store information. Each magnetic memory cell typically comprises a submicron piece of magnetic material, e.g., having the dimensions of 300 nanometers (nm) by 600 nm in area and five nm thick.
Information is stored in such semiconductor devices as the orientation of the magnetization of a free layer in the magnetic memory cell as compared to the orientation of the magnetization of a fixed (e.g., reference) layer in the memory cell. The magnetization of the free layer may be oriented parallel or anti-parallel to the fixed layer, representing either a logic “1” or a “0.” The orientation of the magnetization of a given layer (fixed or free) may be represented by an arrow pointing either to the left or to the right. When the magnetic memory cell is sitting in a zero applied magnetic field, the magnetization of the magnetic memory cell is stable, pointing either left or right. The application of a magnetic field can switch the magnetization of the free layer from left to right, and vice versa, to write information to the magnetic memory cell. One of the important requirements for data storage is that the magnetization of the cell not change orientation when there is a zero applied field, or only a small applied field.
Unfortunately, in practice, the magnetization of one or more magnetic memory cells may change orientation unintentionally, due, at least in part, to thermal activation. Thermal activation occurs when thermal energy from the environment surrounding a given cell overcomes an activation energy barrier so as to change the direction of magnetization of the cell. The occurrences of thermal activation should be minimized. The resulting error rate due to thermally activated switching is called the soft error rate (SER).
One of the objectives in designing MRAM devices is to minimize operating power and area consumed by the devices. Low operating power and small area requires a low switching field for the magnetic memory cell. A low switching field uses a low switching current, which in turn uses less power. Further, lower switching currents require smaller switches, which occupy less area. Consequently, these two design objectives are consistent with one another.
As the area of the magnetic memory cells becomes increasingly smaller, a process generally referred to as “scaling” due to the fact that the cell area is scaled down to increase density, the SER becomes worse. As mentioned above, the activation energy barrier may be overcome due to thermal energy, resulting in thermal activation. Therefore, it is desirable to have a large enough activation energy barrier to prevent thermal activation and to prevent the magnetization of the cell from changing direction unintentionally.
According to single domain theory, the activation energy barrier of the magnetic memory cell is proportional to the volume of the cell. Therefore, as the area is scaled down, assuming nothing else changes, the activation energy barrier decreases and the SER becomes unacceptably large. A conventional, simple solution to this problem would be to increase the thickness of the cell as the area of the cell is scaled down, to thereby maintain a large enough volume to ensure a suitable energy activation barrier level. However, this technique is undesirable, at least in part because a greater magnetic field is required to switch the magnetization of a thicker cell. Thus, a primary goal of the scaling process becomes making the area of the cell smaller, but maintaining the activation energy barrier and the switching field, i.e., preventing the activation energy barrier from becoming too small and preventing the switching field from becoming too large.
U.S. Pat. No. 6,633,498, issued to Engel et al. (hereinafter “Engel”), discloses a method for reducing the write field of a toggle MRAM by adding an easy axis offsetting field. However, while the techniques highlighted in Engel can be employed to reduce the write field, the effects of the offsetting field can result in an increased SER, potentially rendering the cell inoperable.
Therefore, techniques are needed to reduce the magnetic field required to switch a magnetic memory cell while at the same time reducing, or eliminating, the occurrence of soft errors.